1. Field of the Invention
The invention relates to metal deforming, class 72-344, and to metallic can body strippers.
2. Description of the Prior Art
In the metal forming art it has long been known that metal objects can be formed from a metal blank by forcing the blank through a series of ironing dies under the force of a ram. Such a process is used to form the well known one-piece aluminum can body and similar machinery has previously been used to form shell casings as disclosed in U.S. Pat. No. 2,345,857 to Newell, and to draw tubing as disclosed in U.S. Pat. No. 2,369,260 to Slater.
After the metal object has been formed, it is tightly engaged on a punch attached to the end of the ram and must be removed as the ram retracts through the ironing dies. For this purpose, strippers have long been used. A stripper is usually a complete ring composed of wedge-shaped segments machined to fit together with perfectly abutting sides, a resilient retainer ring or garter spring passing around the outer circumference of the segment ring to hold the ring intact using the well known keystone concept. The diameter of the inner opening of the ring is slightly larger than the diameter of the ram so that the ram may pass through the stripper ring without contact, but the inner diameter of the ring must be carefully machined to engage the edge of the thin wall of the metal object on the punch as the ram retracts through the stripper. In this way, the stripper holds the metal object as the ram retracts and thereby strips the object from the punch. However, the shape of the segments is such that the ram and metal blank can deflect the segment edges and pass through the stripper during the ram's forward motion while forming the metal object.
The Slater patent discloses a special contour of each segment that avoids scratching the metal object during the ram's forward motion through the stripper. In addition U.S. Pat. No. 3,664,171 to Paramonoff and U.S. Pat. No. 3,735,628 to Hutchinson teach improved pivoting of segments during forward motion of the ram.
In the prior art, stripper segments have been supplied in matched sets for perfect fit between segments. When the segments are circumferentially adjacent, an exact fit is needed so that no segment scores the punch and so that each segment carries its share of the stripping work. When the stripping edge of the segments wears to the point of needing adjustment in order to properly catch the edge of the metal object being stripped, the job requires that each segment be machined to retain perfect circumferentially adjacent fit between segments of a set while reducing the inner diameter of the set by the precise needed amount. This adjustment task is extremely expensive and exacting.
Another problem in the prior art is that segments often fail at the corners of the stripping edge. When a corner breaks, the segment must be either remachined and accordingly the entire set remachined, or discarded, and accordingly the entire matched set discarded. Thus, the requirement for segments to be circumferentially adjacent or in a matched set leads to excessive costs when a single segment fails. All four of the above named patents call for circumferentially adjacent segments, and the Paramonoff and Hutchinson patents describe an additional retainer ring structure that allows no adjustment for segment wear or damage.
A related problem in the prior art is that segments in a matched set are often individually machined from bar stock. The weakest part of the bar is the outer portion, which is the part used for the segments, while the stronger center of the bar is machined away. As a result, the segments of a matched machined set are overly subject to failure because the weakest part of the bar stock was used for their manufacture. When a segment corner fails, the resulting sharp edge may damage the punch and score numerous formed objects being stripped from the punch before the failure is detected, creating a long reaching problem of quality control.
Obtaining rapid segment pivoting is another problem in the art. The speed of a can body former may be limited by the speed with which the segments can close against the punch after being deflected by the punch and its carried can body. Abutting segments create friction between their adjacent sides when they are deflected on the forward stroke of the ram, and if the segments have not closed when the ram retracts, the formed can body will remain on the punch and a second can will be formed over the first. If a plurality of cans accumulates on the punch, the machine may suffer damage and the machine must be stopped for removal of the cans from the punch.